The present invention relates to Light Emitting Diode (LED) devices.
Conventionally, LED devices made from GaAlAs (gallium-aluminum arsenide) have been manufactured as stated below. First, a P-type GaAlAs cladding layer, a P-type GaAlAs active layer and an N-type GaAlAs cladding layer are grown on a substrate by a liquid phase epitaxial growth method so as to form an epitaxial wafer. The epitaxial wafer is etched or lapped to obtain a specified thickness. After the wafer is cleaned and dried, an electrode material made of gold or Al is spattered on both the front and back surfaces of the wafer. Then, the surface of the waver i.e. the surface of an N-type GaAlAs cladding layer is coated with a resist and pre-hardened. A mask pattern is printed thereon and developed, so that a mask in a specified shape is formed. A portion of the electrode material not covered with the mask is etched off to form an electrode having a specified shape. After that, the mask is cleared off by an organic solvent cleaning, and the electrode etched into the specified shape is alloyed. Then, a PN contact surface formed to be a wafer, i.e., the contact surface between the P-type GaAlAs active layer and the N-type GaAlAs cladding layer, is segmented by mesa etching. The mesa etching is performed for manufacturing LED devices that are required relatively high reliability. The mesa etching is performed as follows First, a mask is formed on the surface of the wafer on which the alloyed electrode was provided. The formation of the mask is conducted by a method almost identical to the method for manufacturing the mask which is formed when the electrode is etched. Then, the wafer provided with the mask thereon is dipped in a mesa etchant, and etched up to the PN contact surface. Thereafter, the mask is removed. Next, a wafer test is conducted for measuring electric characteristics of each chip. As a result of the measurement, when a chip is disqualified, the chip is given marking as a defective chip. Then, the wafer is divided into chips by dicing, among which nondefective chips having no marking are collected as complete LED devices.
Thus-manufactured LED devices are used for displays and optical communications. When the LED devices are used in different applications such as displays and optical communications, acceptance criteria of electric characteristics of the LED devices are different in the wafer test. More particularly, in the case of LED devices for displays, only the lower limit criterion is specified on an optical output of the LED devices, whereas in the case of LED devices for optical communications both the upper and lower limit criteria are specified thereon. As for response characteristics, similarly, only the lower limit criterion is specified for the LED devices for displays, whereas both the upper and lower limit criteria are specified for the LED devices for optical communications.
However, when the conventional LED devices are used for optical communications, they have a problem of relatively high reject rate and low yield. This is because it is difficult to satisfy both the upper and lower limit criteria of the optical output and the response characteristics due to quality of internal luminous efficacy of the LED devices and dispersion of processing in a wafer process.